Let me provide one example from a local news:
"Essentially what happened is the state approved the same dollar amount it did last year for the supplemental general fund"
My thinking is, the above, as well generally, is incorrect b/c of the verb-agreement rules - and "what happened was" would be [more] correct, context depended obviously. However, based in somewhat equal google hits and in speech, I see a prevalent use of "what happened is..."
Please explain when it is correct and when it is NOT.